1. Field of the Invention
The present invention relates to vibratory seismic energy sources used for seismic prospecting. More particularly, it relates to a system for controlling the tilt of a vehicle upon which a seismic vibrator may be mounted.
2. Description of the Prior Art
In seismic prospecting, it has become common to use, as a seismic wave source, an electrohydraulically controlled vibratory source, more simply referred to as a vibrator.
Typically, a vibrator comprises a double-ended piston rigidly affixed to a coaxial piston rod. The piston is located in reciprocating relationship in a cylinder formed within a heavy reaction mass. Means are included for alternately introducing hydraulic fluid under high pressure to opposite ends of the cylinder, thereby imparting a reciprocating motion to the piston relative to the reaction mass. The piston rod extending from the reaction mass is rigidly coupled to a ground coupling plate (hereinafter baseplate) which is maintained in intimate contact with the earth material. The inertia of the reaction mass tends to resist displacement of the reaction mass relative to the earth. The motion of the piston is coupled through the piston rod and baseplate to impart vibratory seismic energy in the earth.
Typically, a vibrator is vehicle mounted. Upon arrival at a selected location, the baseplate is lowered into contact with the earth's surface, and to prevent decoupling of the baseplate from the ground during operation, a portion of the vehicle's weight is applied to the baseplate. The weight of the vehicle is frequently applied to the baseplate through one or more spring members so that a static bias force is imposed on the baseplate while the dynamic forces of the baseplate are decoupled from the vehicle. The spring members are usually in the form of rubber air bags filled with compressed air.
As frequently occurs in seismic exploration, the baseplate will be positioned on sloping terrain such that during operation of the vibrator one side of the vehicle will be at a higher elevation than the other side. Under such conditions, the vehicle and the vibrator will be tilted in the direction of the slope. Where the slope is relatively slight, the tilt produced by the surface irregularity is compensated for by the compliancy of the air bags. At some point, however, the degree of slope will be so severe that the center of gravity of the vehicle will shift far enough in the direction of the tilt so as to cause the vehicle to roll over. The vehicle will tend to remain stable as long as a vertical line drawn through the center of gravity of the vehicle intersects the plane formed by the ground coupling surface of the baseplate. Thus, the likelihood of vehicle roll-over increases as the surface area of the baseplate is reduced.
The current method utilized to prevent vehicle roll over, when the vibrator is being positioned on sloping terrain, is to operate the vibrator with the rear wheels of the vehicle in contact with the earth's surface. This maintains the vibrator in a stable condition, but reduces the bias force applied to the baseplate, which has the undesirable effect of reducing the amount of seismic energy the vibrator is capable of imparting into the earth. This follows from the fact that when the magnitude of the bias force applied to the baseplate is reduced, a smaller upward force will result in decoupling of the baseplate from the earth's surface. When the baseplate is not in contact with the earth's surface, seismic energy can not be imparted into the earth. Decoupling also produces distortion in the injected energy pattern.